1. Field of the Invention
The present invention relates to a vector controller of an induction motor, and in particular, to an induction motor controller capable of detecting a speed without connecting a mechanical speed detector or a position detector to the induction motor.
2. Description of the Prior Art
FIG. 1 is a block diagram showing a conventional induction motor controller, for example, described in the "Induction Motor Control Method employing Slip Frequency Control Type without Speed Detector", the Institute of Electrical Engineers of Japan, Proceedings of Semiconductor Power Conversion Meeting (SPC-84-61). The configuration of FIG. 1 includes an induction motor 1, a three phase power amplifier 2, and a coordinate converter 3 for converting voltage instructions Vd.sup.e s* and Vq.sup.e s* into a three phase voltage whose angular frequency is .omega.. The power amplifier 2 and the coordinate converter 3 constitute a power supply unit. The system further comprises a coordinate converter 4 for converting an alternating current into a direct current viewed on orthogonal coordinate axes (d.sup.e axis and q.sup.e axis) rotating at an angular frequency .omega., a circuit 5 for computing from the alternating current the primary interlinkage flux .lambda.d.sup.e s*, .lambda.q.sup.e s* and the primary voltage Vq.sup.e s on the q.sup.e axis, an adder 6, constant multipliers 7 to 9, subtractors 10 to 12, an arithmetic unit 13 for removing an interference from the quantity of state on the q.sup.e axis onto the d.sup.e axis, an arithmetic unit 14 for computing the primary angular frequency .omega., an integrator 15, a trigonometric function generator 16, PI compensators 17 and 18, and subtractors 19 and 20.
Next, the operation will be described. The induction motor 1 is driven by the three phase power amplifier 2. The equation of the induction motor 1 is expressed as follows. ##EQU1## In this equation, R.sub.s and R.sub.r respectively indicate a primary resistance and a secondary resistance, L.sub.s, L.sub.r, and M respectively represent a primary inductance, a secondary inductance, and a mutual inductance, .sigma. stands for a leakage coefficient, p is a number of pole pairs, P=d/dt is a differential operator, Vd.sup.e s and Vq.sup.e s respectively are primary voltages for the d.sup.e and q.sup.e axes, id.sup.e s and iq.sup.e s respectively designate primary currents for the d.sup.e and q.sup.e axes, .lambda.d.sup.e r and .lambda.q.sup.e r respectively are secondary interlinkage flux for the d.sup.e and q.sup.e axes, .omega..sub.r stands for an angular velocity of a rotor, and .omega. is a primary angular frequency.
In the vector control, the coordinate axes d.sup.e and q.sup.e are required to be controlled so as to develop .lambda.q.sup.e r=0. For this purpose, the control equation concerning Vd.sup.e s, Vq.sup.e s, and .omega. is obtained as follows.
Expression (2) is attained by transforming the first and second rows of the expression (1). ##EQU2## Where, .lambda.d.sup.e s=.sigma.L.sub.s id.sup.e s+M.lambda.d.sup.e r/L.sub.r and .lambda.q.sup.e s=.sigma.L.sub.s iq.sup.e s+M.lambda.q.sup.e r/L.sub.r are the primary interlinkage flux with respect to the axes d.sup.e and q.sup.e. Assuming that the fluxes .lambda.d.sup.e s and .lambda.q.sup.e s are detected, when Vd.sup.e s and Vq.sup.e s are represented as follows, ##EQU3## expression (4) is obtained. ##EQU4## Controlling .omega. by use of expression (5), the second row of the expression (4) is reduced to expression (6). ##EQU5## From the expression (6), it is clear that .lambda.q.sup.e r converges to 0 under a condition T.sub.2 &gt;0.
As described above, when .lambda.d.sup.e s and .lambda.q.sup.e s are detected to accomplish the control according to the expressions (3) and (5), the vector control of the induction motor can be effected without using a speed detector. That is, the control of the expression (3) is achieved by use of the constant multipliers 8 and 9 and the control of the expression (5) is carried out with the arithmetic unit 14 associated with the primary angular frequency .omega.. In addition, the primary angular frequency .omega. outputted from the arithmetic unit 14 is integrated by the integrator 15 to attain a phase signal such that the trigonometric wave generator 16 receiving the phase signal outputs the sine and cosine values thereof.
Furthermore, in the expression (1), in order to remove the interference component from the q.sup.e axis onto the d.sup.e axis, the arithmetic units 13 and 14 achieved a control according to EQU V'd.sup.e s=V"d.sup.e s-.omega..sigma.Ls iq.sup.e s (7)
The currents id.sup.e s and iq.sup.e s associated with the d.sup.e and q.sup.e axes are subjected to a feedback control by means of the PI compensator 17 and the subtractor 19 and the PI compensator 18 and the subtractor 20, respectively.
The primary interlinkage fluxes .lambda.d.sup.e s and .lambda.q.sup.e s for the d.sup.e and q.sup.e axes are detected according to the expression (2). The block diagram of this detection method is, for example, as shown in FIG. 2 in which integrators 29 and 30 are required and a complete integrator cannot be implemented; consequently, a primary delay filter of a time lag of first order is employed therefor as shown in FIG. 3.
Since the conventional induction motor controller not using a speed detector is constituted as described above, the operation accuracy of .lambda.d.sup.e s and .lambda.q.sup.e s is deteriorated particularly in the low-speed region due to the utilization of the filter of the time lag of first order, which leads to the occurrence of estimation resulting error of .omega. and the error in the generated torque.